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Abstract:

In the optical receiver available for the RZ-DPSK modulation system, the
difference in the received intensity due to the difference in the
intensity or optical path of the optical signal cannot be corrected
automatically, therefore, an optical receiver according to an exemplary
aspect of the invention includes a first photodiode receiving a normal
phase optical signal from a first output of a 1-bit delayed
interferometer and outputting a positive signal; a second photodiode
receiving a reversed phase optical signal from a second output of the
1-bit delayed interferometer and outputting a complementary signal; a
differential transimpedance amplifier inputting the positive signal and
the complementary signal and including a closed feedback loop for each
input of the positive signal and the complementary signal; a level
adjustment unit adjusting a signal level in the closed feedback loop; a
photoelectric current detection unit detecting a photoelectric current
generated in each of the first photodiode and the second photodiode; and
wherein the level adjustment unit adjusts the signal level on the basis
of an output of the photoelectric current detection unit.

Claims:

1. An optical receiver, comprising: a first photodiode receiving a normal
phase optical signal from a first output of a 1-bit delayed
interferometer and outputting a positive signal; a second photodiode
receiving a reversed phase optical signal from a second output of the
1-bit delayed interferometer and outputting a complementary signal; a
differential transimpedance amplifier inputting the positive signal and
the complementary signal and comprising a closed feedback loop for each
input of the positive signal and the complementary signal; a level
adjustment unit adjusting a signal level in the closed feedback loop; a
photoelectric current detection unit detecting a photoelectric current
generated in each of the first photodiode and the second photodiode; and
wherein the level adjustment unit adjusts the signal level on the basis
of an output of the photoelectric current detection unit.

2. The optical receiver according to claim 1, wherein the level
adjustment unit adjusts a signal level in the closed feedback loop for
the input of the complementary signal on the basis of the photoelectric
current generated by the normal phase optical signal, and adjusts a
signal level in the closed feedback loop for the input of the positive
signal on the basis of the photoelectric current generated by the
reversed phase optical signal.

3. The optical receiver according to claim 1, wherein the differential
transimpedance amplifier comprises a differential amplifier inputting the
positive signal and the complementary signal, and any one of an
emitter-follower circuit and a source-follower circuit connected to an
output of the differential amplifier; the closed feedback loop comprises
a feedback resister connected between an input of the differential
amplifier and an output of any one of the emitter-follower circuit and
the source-follower circuit; the level adjustment unit comprises a
differential circuit connected between the output of the differential
amplifier and an input of any one of the emitter-follower circuit and the
source-follower circuit; and the photoelectric current detection unit
comprises a current mirror circuit outputting a proportional current
proportional to the photoelectric current generated in each of the first
photodiode and the second photodiode, and an adjustment voltage
generation unit generating an adjustment voltage on the basis of the
proportional current, and wherein the adjustment voltage is inputted into
an input of the differential circuit in reverse between positive and
complementary signals.

4. The optical receiver according to claim 1, wherein the level
adjustment unit adjusts a signal level in the closed feedback loop for
the input of the positive signal on the basis of an inverted value of a
voltage which is generated depending on the photoelectric current
generated by the normal phase optical signal, and adjusts a signal level
in the closed feedback loop for the input of the complementary signal on
the basis of an inverted value of a voltage which is generated depending
on the photoelectric current generated by the reversed phase optical
signal.

5. The optical receiver according to claim 1, wherein the differential
transimpedance amplifier comprises a differential amplifier inputting the
positive signal and the complementary signal, and any one of an
emitter-follower circuit and a source-follower circuit connected to an
output of the differential amplifier; the closed feedback loop comprises
a feedback resister connected between an input of the differential
amplifier and an output of any one of the emitter-follower circuit and
the source-follower circuit; the level adjustment unit comprises a
differential circuit connected between the output of the differential
amplifier and an input of any one of the emitter-follower circuit and the
source-follower circuit; and the photoelectric current detection unit
comprises a current mirror circuit outputting a proportional current
proportional to the photoelectric current generated in each of the first
photodiode and the second photodiode, an adjustment voltage generation
unit generating an adjustment voltage on the basis of the proportional
current, and an inverter circuit outputting a level adjustment voltage
obtained by inverting the adjustment voltage and amplifying an inverted
adjustment voltage, and wherein the level adjustment voltage is inputted
into the differential circuit.

6. An optical reception device, comprising: the optical receiver
according to claim 1; a 1-bit delayed interferometer; and wherein the
1-bit delayed interferometer receives an optical modulated signal based
on the differential phase shift keying modulation using the
return-to-zero code, and outputs the normal phase optical signal and the
reversed phase optical signal.

7. A correction method for optical received intensity, comprising the
steps of: receiving a normal phase optical signal from a first output of
a 1-bit delayed interferometer and outputting a positive signal;
receiving a reversed phase optical signal from a second output of the
1-bit delayed interferometer and outputting a complementary signal;
inputting the positive signal and the complementary signal, and
outputting a positive signal voltage and a complementary signal voltage,
and feeding back the positive signal voltage and the complementary signal
voltage to an input side; detecting a photoelectric current generated by
each of the normal phase optical signal and the reversed phase optical
signal; and adjusting a signal level in feedback on the basis of the
photoelectric current.

8. The correction method for optical received intensity according to
claim 7, wherein, in the step of adjusting the signal level in feedback,
adjusting the signal level in feedback for the input of the complementary
signal on the basis of the photoelectric current generated by the normal
phase optical signal; and adjusting the signal level in feedback for the
input of the positive signal on the basis of the photoelectric current
generated by the reversed phase optical signal.

9. The correction method for optical received intensity according to
claim 7, wherein, in the step of adjusting the signal level in feedback,
adjusting the signal level in feedback for the input of the positive
signal on the basis of an inverted value of a voltage which is generated
depending on the photoelectric current generated by the normal phase
optical signal; and adjusting the signal level in feedback for the input
of the complementary signal on the basis of an inverted value of a
voltage which is generated depending on the photoelectric current
generated by the reversed phase optical signal.

10. A coherent optical receiver, comprising: a first photodiode receiving
a first interference optical signal obtained by making an optical
reception signal interfere with a first local oscillation light whose
wave length is almost the same as that of the optical reception signal,
and outputting a positive signal; a second photodiode receiving a second
interference optical signal obtained by making the optical reception
signal interfere with a second local oscillation light whose phase is
reverse to that of the first local oscillation light, and outputting a
complementary signal; a differential transimpedance amplifier inputting
the positive signal and the complementary signal and comprising a closed
feedback loop for each input of the positive signal and the complementary
signal; a level adjustment unit adjusting a signal level in the closed
feedback loop; a photoelectric current detection unit detecting a
photoelectric current generated in each of the first photodiode and the
second photodiode, and wherein the level adjustment unit adjusts the
signal level on the basis of an output of the photoelectric current
detection unit.

11. The coherent optical receiver according to claim 10, wherein the
level adjustment unit adjusts a signal level in the closed feedback loop
for the input of the complementary signal on the basis of the
photoelectric current generated by the first interference optical signal,
and adjusts a signal level in the closed feedback loop for the input of
the positive signal on the basis of the photoelectric current generated
by the second interference optical signal.

12. The coherent optical receiver according to claim 10, wherein the
differential transimpedance amplifier comprises a differential amplifier
inputting the positive signal and the complementary signal, and any one
of an emitter-follower circuit and a source-follower circuit connected to
an output of the differential amplifier; the closed feedback loop
comprises a feedback resister connected between an input of the
differential amplifier and an output of any one of the emitter-follower
circuit and the source-follower circuit; the level adjustment unit
comprises a differential circuit connected between the output of the
differential amplifier and an input of any one of the emitter-follower
circuit and the source-follower circuit; and the photoelectric current
detection unit comprises a current mirror circuit outputting a
proportional current proportional to the photoelectric current generated
in each of the first photodiode and the second photodiode, and an
adjustment voltage generation unit generating an adjustment voltage on
the basis of the proportional current, and wherein the adjustment voltage
is inputted into an input of the differential circuit in reverse between
positive and complementary signals.

13. The coherent optical receiver according to claim 10, wherein the
level adjustment unit adjusts a signal level in the closed feedback loop
for the input of the positive signal on the basis of an inverted value of
a voltage which is generated depending on the photoelectric current
generated by the first interference optical signal, and adjusts a signal
level in the closed feedback loop for the input of the complementary
signal on the basis of an inverted value of a voltage which is generated
depending on the photoelectric current generated by the second
interference optical signal.

14. The coherent optical receiver according to claim 10, wherein the
differential transimpedance amplifier comprises a differential amplifier
inputting the positive signal and the complementary signal, and any one
of an emitter-follower circuit and a source-follower circuit connected to
an output of the differential amplifier; the closed feedback loop
comprises a feedback resister connected between an input of the
differential amplifier and an output of any one of the emitter-follower
circuit and the source-follower circuit; the level adjustment unit
comprises a differential circuit connected between the output point of
the differential amplifier and an input of any one of the
emitter-follower circuit and the source-follower circuit; and the
photoelectric current detection unit comprises a current mirror circuit
outputting a proportional current proportional to the photoelectric
current generated in each of the first photodiode and the second
photodiode, an adjustment voltage generation unit generating an
adjustment voltage on the basis of the proportional current, and an
inverter circuit outputting a level adjustment voltage obtained by
inverting the adjustment voltage and amplifying an inverted adjustment
voltage, and wherein the level adjustment voltage is inputted into the
differential circuit.

15. The coherent optical receiver according to claim 10, wherein the
differential transimpedance amplifier comprises a differential amplifier
inputting the positive signal and the complementary signal, and any one
of an emitter-follower circuit and a source-follower circuit connected to
an output of the differential amplifier; the closed feedback loop
comprises a feedback resister connected between an input of the
differential amplifier and an output of any one of the emitter-follower
circuit and the source-follower circuit; the level adjustment unit
comprises a differential circuit connected to an output part of any one
of the emitter-follower circuit and the source-follower circuit; and the
photoelectric current detection unit comprises, a current mirror circuit
outputting a proportional current proportional to the photoelectric
current generated in each of the first photodiode and the second
photodiode, an adjustment voltage generation unit generating an
adjustment voltage on the basis of the proportional current, and an
inverter circuit outputting a level adjustment voltage obtained by
inverting the adjustment voltage and amplifying an inverted adjustment
voltage, and wherein the level adjustment voltage is inputted into the
differential circuit.

16. A coherent optical reception device, comprising: the coherent optical
receiver according to claim 10; an optical 90 degrees hybrid circuit; and
wherein the optical 90 degrees hybrid circuit outputs the first
interference optical signal by making the optical reception signal
interfere with the first local oscillation light, and outputs the second
interference optical signal by making the optical reception signal
interfere with the second local oscillation light.

17. A correction method for coherent optical received intensity,
comprising the steps of: receiving a first interference optical signal
obtained by making an optical reception signal interfere with a first
local oscillation light whose wave length is almost the same as that of
the optical reception signal, and outputting a positive signal converted
into an electric signal; receiving a second interference optical signal
obtained by making the optical reception signal interfere with a second
local oscillation light whose phase is reverse to that of the first local
oscillation light, and outputting a complementary signal converted into
an electric signal; inputting the positive signal and the complementary
signal, and outputting a positive signal voltage and a complementary
signal voltage, and feeding back the positive signal voltage and the
complementary signal voltage to an input side; detecting a photoelectric
current generated by each of the first interference optical signal and
the second interference optical signal; and adjusting a signal level in
feedback on the basis of the photoelectric current.

18. The correction method for coherent optical received intensity
according to claim 17, wherein, in the step of adjusting the signal level
in feedback, adjusting the signal level in feedback for the input of the
complementary signal on the basis of the photoelectric current generated
by the first interference optical signal; and adjusting the signal level
in feedback for the input of the positive signal on the basis of the
photoelectric current generated by the second interference optical
signal.

19. The correction method for coherent optical received intensity
according to claim 17, wherein, in the step of adjusting the signal level
in feedback, adjusting the signal level in feedback for the input of the
positive signal on the basis of an inverted value of a voltage which is
generated depending on the photoelectric current generated by the first
interference optical signal; and adjusting the signal level in feedback
for the input of the complementary signal on the basis of an inverted
value of a voltage which is generated depending on the photoelectric
current generated by the second interference optical signal.

Description:

TECHNICAL FIELD

[0001] The present invention relates to optical receivers, optical
reception devices, and correction methods for optical received intensity,
and in particular, to an optical receiver, an optical reception device,
and a correction method for optical received intensity which are
available for DPSK (Differential Phase Shift Keying) system or DQPSK
(Differential Quadrature Phase Shift Keying) system using the RZ
(Return-to-Zero) code as a modulation code.

[0002] Moreover, the present invention relates to coherent optical
receivers, and in particular, to a coherent optical receiver which is
available for QPSK (Quadrature Phase Shift Keying) system as a modulation
code.

BACKGROUND ART

[0003] The long-haul optical transmission system realizes economical and
large-volume information transmission by applying the WDM (Wavelength
Division Multiplexing) transmission technology which multiplexes a
plurality of optical signals with various wavelengths and transmits them
through one optical fiber. In order to reduce a cost of the WDM
transmission device, a transmission speed per one wavelength is upgraded
to high speed. The transmission speed of 10 gigabits per second (Gbit/s)
for each wavelength is put into practical use currently, and furthermore
a transmission technology for 40 Gbit/s and 100 Gbit/s has been studied.

[0004] When the transmission rate is speeded up to 40 Gbit/s and 100
Gbit/s from 10 Gbit/s, it becomes a main problem to improve the optical
noise tolerance, that is, SN ratio (Signal to Noise ratio). In other
words, in the case of long-haul transmission, the transmission length is
generally limited due to the optical noise arising in an optical
amplifier which is used on a transmission line and in an optical
transmitter and receiver. Therefore, if the same modulation and
demodulation system as that for 10 Gbit/s transmission rate is used for
40 Gbit/s transmission rate, the noise tolerance is reduced to a quarter.
For this reason, in the case of the 40 Gbit/s transmission rate, it is
necessary to adopt a modulation and demodulation system with the strong
optical noise tolerance. The configuration is currently a typical system
where RZ-DPSK system or RZ-DQPSK system is applied as the
modulation/demodulation system and a balanced receiver using a delayed
interferometer is applied in the receiving side.

[0005] An example of the above-mentioned optical reception device is
described in patent literature 1. FIG. 12 shows the configuration of the
related optical reception device 600 described in the patent literature
1. The optical reception device 600, which demodulates the RZ-DPSK
signal, includes a related optical receiver 610 and a related 1-bit
delayed interferometer 650. The 1-bit delayed interferometer 650 is
provided with a 1-bit delay element in one optical waveguide of a set of
optical waveguides, and outputs a set of two optical signals 652 and 653
which correspond to a phase difference between adjacent bits of one
optical input signal 651.

[0006] The optical receiver 610 includes two photodiodes (PD) 611 and 612
and a transimpedance amplifier 620. The photodiodes (PD) 611 and 612
convert two optical signals outputted from the 1-bit delayed
interferometer 650 into intensity modulated signals. The transimpedance
amplifier 620 is provided with a differential amplifier with a
differential negative feedback 622 and is connected to the photodiodes
(PD) 611 and 612. The transimpedance amplifier 620 obtains the intensity
modulated signals from the photodiodes (PD) 611 and 612 and demodulates
the RZ-DPSK signal through outputting the difference between them.

[0007] In the related optical reception device available for the RZ-DPSK
modulation, it is necessary that the phase difference by 1 bit between
two signals is accurately kept and that the intensities of the signals
are equal until the demodulation is carried out. However, the received
intensity of two optical signals may not be kept equal in some cases due
to the difference in the intensity or optical path between two optical
signals on the path through which the optical inputting signal passes
through the 1-bit delayed interferometer and lenses and then is inputted
into the photodiode. The difference in the received intensity of these
signals degrades CMRR (Common Mode Rejection Ratio) and causes waveform
distortion and an increase of jitter after the demodulation. Moreover, it
is difficult to control the optical received intensity mentioned above
with a high degree of accuracy.

[0008] A technology for solving those problems is described in the patent
literature 1. As shown in FIG. 13, another related optical reception
device 700 described in the patent literature 1 is provided with a
related optical receiver 710 and the 1-bit delayed interferometer 650.
The optical receiver 710 includes two photodiodes (PD) 711 and 712, a
transimpedance amplifier 720 with a differential negative feedback, and a
level adjustment unit 730. The transimpedance amplifier 720 is provided
with a differential amplifier 721 with a differential negative feedback
722, and is connected to the photodiodes (PD) 711 and 712. The level
adjustment unit 730 is connected to the transimpedance amplifier 720 and
has a function of adjusting the levels of positive and complementary
signals in two closed feedback loops. By adjusting the levels of positive
and complementary signals in two closed feedback loops, the difference in
the intensities between two signals before demodulation is corrected.

[0009] On the other hand, the coherent detection system is well known
where the detection is performed by mixing a signal light with a
reference light and detecting an interfering signal (beat signal) which
is generated by the mixture. FIG. 14 shows an example of the
configuration of a related coherent optical reception device which is
applied to the coherent detection system. The related coherent optical
reception device 5000 receives an optical reception signal 5001 and a
local oscillation light 5002 whose wavelength is almost equal to the
optical reception signal 5001 from a local oscillation light source, and
makes the local oscillation light 5002 and the optical reception signal
5001 interfere each other, and converts the interference signal into an
electric signal (coherent detection). Since the coherent detection system
has strong dependency on polarization, one optical receiver receives only
an optical signal whose polarization state is identical to that of the
local oscillation light. Then, the related coherent optical reception
device 5000 is provided with a polarization demultiplexing unit 5010 at
the input part of the optical reception signal 5001. The polarization
demultiplexing unit 5010 demultiplexes the optical reception signal 5001
into two orthogonal polarization components. As a result, although it is
necessary to use two optical receivers in order to receive one optical
signal, this disadvantage can be compensated by making an amount of
transmission information two times larger using polarization multiplexing
scheme.

[0010] Each polarization light of the optical reception signal 5001 and
the local oscillation light 5002 are inputted into an optical 90 degrees
hybrid circuit 5100. The optical 90 degrees hybrid circuit 5100 outputs
four kinds of output light in total, that is, a pair of output light
which are obtained by making each polarization light and the local
oscillation light interfere in normal phase and reversed phase, and
another pair of output light which are obtained by making each
polarization light and the local oscillation light interfere in
quadrature phase (90 degrees) and inverted quadrature phase (-90
degrees). These output optical signals are converted into current signals
by two photodiodes 5200 for a pair of output light, and then are inputted
into a differential trans impedance amplifier 5300. Since their direct
current components are balanced (canceled) consequently, it is possible
to extract efficiently only the beat components between the optical
reception signal 5001 and the local oscillation light 5002. The
electrical signals outputted from the differential transimpedance
amplifier 5300 correspond to an in-phase component (I component) and a
quadrature component (Q component) of the interference between the
optical reception signal and the local oscillation light, respectively.

[0011] The output signals for every polarization, that is, four kinds of
the electrical signals in total which are composed of the I component and
the Q component of X polarization and the I component and the Q component
of Y polarization, are converted very fast from analog signals to digital
signals by an analog-to-digital conversion unit (ADC) 5400, respectively.
The electrical signal is converted into the digital information signal
and then is inputted into a digital signal processing unit (DSP) 5500. It
becomes possible to carry out various equalization and decision processes
on the above-mentioned digital signal by applying the digital signal
processing technology which is widely used in the field of the wireless
communication. After carrying out the digital signal processing and the
error correction processing, the super high speed (for example, 100
Gbit/s) information signal is outputted. Patent Literature 1: WO
2009/069814 (FIG. 1 and FIG. 11)

DISCLOSURE OF INVENTION

Problem to be Solved by the Invention

[0012] In the above-mentioned related optical reception device 700, the
intensity difference between two signals before demodulation is corrected
by adjusting the levels of positive and complementary signals in two
closed feedback loops. At this time, in order to adjust the levels of
positive and complementary signals in the feedback loops, it is necessary
to observe a waveform of the demodulated signal. Therefore, there is a
problem that it is impossible to correct the level adjustment
automatically. As mentioned above, in the related optical receiver
available for the RZ-DPSK modulation system, there is a problem that the
difference in the received intensity due to the difference in the
intensity or optical path of the optical signal cannot be corrected
automatically.

[0013] On the other hand, with respect to the related coherent optical
reception device 5000, the common mode rejection ratio (CMRR) for the
optical input into the photodiode 5200 is one of the most important
factors which determine the performance required for the coherent optical
receiver. The CMRR is expressed in the following formula, where
photoelectric currents generated by two photodiodes are represented by
I1 and I2, respectively.

C M R R = 20 log I 1 - I 2
I 1 + I 2 ##EQU00001##

[0014] Accordingly, the CMRR is degraded owing to the difference in the
received intensity generated by the difference in the intensity or
optical path of the optical signal. If the CMRR is degraded, surplus
components of a pulse repetition frequency of the local oscillation light
and its higher harmonics make the transimpedance amplifier saturated, and
make its linearity decrease. As a result, it is difficult to equalize the
waveform distortion accurately in the digital signal processing at the
subsequent stage. However, the related coherent optical receiver has a
problem that the difference in the received intensity due to the
difference in the intensity or optical path of the optical signal cannot
be corrected automatically.

[0015] An object of the present invention is to provide an optical
receiver, an optical reception device, and a correction method for
optical received intensity which are able to solve the problem that in
the optical receiver available for the RZ-DPSK modulation system, the
difference in the received intensity due to the difference in the
intensity or optical path of the optical signal cannot be corrected
automatically.

[0016] Moreover, an object of the present invention is to provide a
coherent optical receiver which is able to solve the problem that in the
related coherent optical receiver, the difference in the received
intensity due to the difference in the intensity or optical path of the
optical signal cannot be corrected automatically.

Means for Solving a Problem

[0017] An optical receiver according to an exemplary aspect of the
invention includes a first photodiode receiving a normal phase optical
signal from a first output of a 1-bit delayed interferometer and
outputting a positive signal; a second photodiode receiving a reversed
phase optical signal from a second output of the 1-bit delayed
interferometer and outputting a complementary signal; a differential
transimpedance amplifier inputting the positive signal and the
complementary signal and including a closed feedback loop for each input
of the positive signal and the complementary signal; a level adjustment
unit adjusting a signal level in the closed feedback loop; a
photoelectric current detection unit detecting a photoelectric current
generated in each of the first photodiode and the second photodiode; and
wherein the level adjustment unit adjusts the signal level on the basis
of an output of the photoelectric current detection unit.

[0018] A correction method for optical received intensity according to an
exemplary aspect of the invention includes the steps of receiving a
normal phase optical signal from a first output of a 1-bit delayed
interferometer and outputting a positive signal; receiving a reversed
phase optical signal from a second output of the 1-bit delayed
interferometer and outputting a complementary signal; inputting the
positive signal and the complementary signal, and outputting a positive
signal voltage and a complementary signal voltage, and feeding back the
positive signal voltage and the complementary signal voltage to an input
side; detecting a photoelectric current generated by each of the normal
phase optical signal and the reversed phase optical signal; and adjusting
a signal level in feedback on the basis of the photoelectric current.

[0019] A coherent optical receiver according to an exemplary aspect of the
invention includes a first photodiode receiving a first interference
optical signal obtained by making an optical reception signal interfere
with a first local oscillation light whose wave length is almost the same
as that of the optical reception signal, and outputting a positive
signal; a second photodiode receiving a second interference optical
signal obtained by making the optical reception signal interfere with a
second local oscillation light whose phase is reverse to that of the
first local oscillation light, and outputting a complementary signal; a
differential transimpedance amplifier inputting the positive signal and
the complementary signal and including a closed feedback loop for each
input of the positive signal and the complementary signal; a level
adjustment unit adjusting a signal level in the closed feedback loop; a
photoelectric current detection unit detecting a photoelectric current
generated in each of the first photodiode and the second photodiode, and
wherein the level adjustment unit adjusts the signal level on the basis
of an output of the photoelectric current detection unit.

[0020] A correction method for coherent optical received intensity
according to an exemplary aspect of the invention includes the steps of
receiving a first interference optical signal obtained by making an
optical reception signal interfere with a first local oscillation light
whose wavelength is almost the same as that of the optical reception
signal, and outputting a positive signal converted into an electric
signal; receiving a second interference optical signal obtained by making
the optical reception signal interfere with a second local oscillation
light whose phase is reverse to that of the first local oscillation
light, and outputting a complementary signal converted into an electric
signal; inputting the positive signal and the complementary signal, and
outputting a positive signal voltage and a complementary signal voltage,
and feeding back the positive signal voltage and the complementary signal
voltage to an input side; detecting a photoelectric current generated by
each of the first interference optical signal and the second interference
optical signal; and adjusting a signal level in feedback on the basis of
the photoelectric current.

Effect of the Invention

[0021] According to the optical receiver of the present invention, in the
optical receiver available for the RZ-DPSK modulation system, it becomes
possible to correct automatically the difference in the received
intensity due to the difference in the intensity or optical path of the
optical signal.

[0022] Moreover, according to the coherent optical receiver of the present
invention, it becomes possible to correct automatically the difference in
the received intensity due to the difference in the intensity or optical
path of the optical signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 is a block diagram showing the configuration of an optical
reception device according to the first exemplary embodiment according to
the present invention.

[0024]FIG. 2 is a circuit diagram showing the configuration of an optical
receiver according to the second exemplary embodiment of the present
invention.

[0025]FIG. 3 is a circuit diagram to illustrate the operation of the
optical receiver according to the second exemplary embodiment of the
present invention.

[0026]FIG. 4 is a circuit diagram showing the configuration of an optical
receiver according to the third exemplary embodiment of the present
invention.

[0027]FIG. 5 is a waveform chart showing the signal waveforms after
RZ-DPSK demodulation carried out by the optical reception device
according to the exemplary embodiments of the present invention.

[0028] FIG. 6 is a block diagram showing the configuration of a coherent
optical reception device according to the fourth exemplary embodiment of
the present invention.

[0029] FIG. 7 is a circuit diagram showing the configuration of a coherent
optical receiver according to the fifth exemplary embodiment of the
present invention.

[0030]FIG. 8 is a circuit diagram to illustrate the operation of the
coherent optical receiver according to the fifth exemplary embodiment of
the present invention.

[0031]FIG. 9 is a waveform chart showing the signal waveforms after QPSK
demodulation in the case of using the coherent optical receiver according
to the exemplary embodiments of the present invention.

[0032] FIG. 10 is a circuit diagram showing the configuration of a
coherent optical receiver according to the sixth exemplary embodiment of
the present invention.

[0033] FIG. 11 is a circuit diagram showing another configuration of the
coherent optical receiver according to the sixth exemplary embodiment of
the present invention.

[0034]FIG. 12 is a block diagram showing the configuration of a related
optical reception device.

[0035]FIG. 13 is a block diagram showing the configuration of another
related optical reception device.

[0036]FIG. 14 is a block diagram showing the configuration of a related
coherent optical reception device.

[0037]FIG. 15 is a waveform chart showing the signal waveforms after QPSK
demodulation carried out by a related coherent optical reception device.

DESCRIPTION OF EMBODIMENTS

[0038] The exemplary embodiments of the present invention will be
described with reference to drawings below.

The First Exemplary Embodiment

[0039]FIG. 1 is a block diagram showing the configuration of an optical
reception device 100 according to the first exemplary embodiment of the
present invention. The optical reception device 100 receives an optical
modulated signal which is obtained by carrying out the differential phase
shift keying (for example, DPSK system or DQPSK system) by using the
return-to-zero (RZ) code (hereinafter, referred to as "RZ-DPSK signal"),
and demodulates the received optical modulated signal. The optical
reception device 100 includes a 1-bit delayed interferometer 200 and an
optical receiver 300.

[0040] The 1-bit delayed interferometer 200 is provided with a 1-bit delay
element in one of a pair of optical waveguides, and outputs a pair of two
optical signals of a first optical signal 221 and a second optical signal
222 which correspond to a phase difference between adjacent bits of one
optical input signal 210 modulated by the RZ-DPSK system. Here, it is
assumed that the first optical signal 221 with a normal phase is
outputted from a first output terminal of the 1-bit delayed
interferometer 200 and the second optical signal 222 with a reversed
phase is outputted from a second output terminal.

[0041] The optical receiver 300 includes a first photodiode 301, a second
photodiode 302, a differential transimpedance amplifier 310, a level
adjustment unit 320, and a photoelectric current detection unit 330.

[0042] The first photodiode 301 receives the first optical signal 221 with
the normal phase from the first output terminal of the 1-bit delayed
interferometer 200 and outputs a positive signal. The second photodiode
302 receives the second optical signal 222 with the reversed phase from
the second output terminal of the 1-bit delayed interferometer 200 and
outputs a complementary signal.

[0043] The differential transimpedance amplifier 310 inputs the positive
signal from the first photodiode 301 and inputs the complementary signal
from the second photodiode 302. Moreover, the differential transimpedance
amplifier 310 is provided with a feedback resister 311 which composes a
closed feedback loop for the positive signal input, and a feedback
resister 312 which composes a closed feedback loop for the complementary
signal input, respectively.

[0044] The photoelectric current detection unit 330 detects photoelectric
currents which are generated by the first photodiode 301 and the second
photodiode 302, respectively. And then, the level adjustment unit 320
adjusts signal levels in the closed feedback loops on the basis of the
output of the optical current detection unit 330.

[0045] Next, the operation of the optical reception device 100 will be
described. If the optical input signals 210 modulated by the RZ-DPSK
system are inputted into the 1-bit delayed interferometer 200, the 1-bit
delayed interferometer 200 outputs the first optical signal 221 and the
second optical signal 222 whose phase difference corresponds to the phase
difference between two adjacent bits. The first optical signal 221 and
the second optical signal 222 are inputted into the first photodiode 301
and the second photodiode 302 respectively, and are converted into
current intensity modulated signals by photoelectric conversion. The
converted current signals are inputted into the differential
transimpedance amplifier 310 which has the negative feedback loops, and
then are converted from the current signals into voltage signals. The
differential transimpedance amplifier 310 demodulates the input signal by
deriving the difference between two input signals, and outputs two
RZ-DPSK demodulated signals (OUT and OUTB) which have a positive and
complementary relationship.

[0046] At this time, if there is a difference in the intensity between two
optical input signals 221 and 222 composing a pair of two signals before
demodulation, the difference appears as a difference between the
photoelectric currents in the first photodiode 301 and the second
photodiode 302.

[0047] The optical reception device 100 of the present exemplary
embodiment has the configuration of detecting the photoelectric currents
generated by two photodiodes 301 and 302 by using the optical current
detection unit 330, and feeding them back to the level adjustment unit
320 connected to the differential transimpedance amplifier 310. By using
the configuration, the signal levels of positive and complementary
signals in two closed feedback loops are adjusted and the difference in
the intensity in a pair of two optical input signals 221 and 222 is
corrected automatically. As a result, two RZ-DPSK demodulated signals are
obtained where the difference in the intensity between two signals before
the demodulation is corrected. FIG. 5 shows signal waveforms after
RZ-DPSK demodulation carried out by the optical reception device 100
according to the present exemplary embodiment. As shown clearly in the
figure, even if the difference in the intensity arises between the
optical signals before the demodulation, it is possible to correct
automatically the difference in the intensity between the optical
signals, and to obtain the demodulated signal waveform with good quality.

[0048] As described above, according to the present exemplary embodiment,
in the optical reception device available for the RZ-DPSK modulation
system, it becomes possible to correct automatically the difference in
the received intensity which arises from the difference in the intensity
and the optical path of the optical signal.

The Second Exemplary Embodiment

[0049] Next, the second exemplary embodiment of the present invention will
be described. FIG. 2 is a circuit diagram showing the configuration of an
optical receiver 400 according to the second exemplary embodiment of the
present invention. The optical receiver 400 includes a first photodiode
401, a second photodiode 402, a differential transimpedance amplifier
410, a level adjustment unit 420, and a photoelectric current detection
unit 430. The configuration is the same as that of the optical receiver
300 according to the first exemplary embodiment. Here, the optical
receiver 400 composes an optical reception device with the 1-bit delayed
interferometer 200.

[0050] The first photodiode 401 and the second photodiode 402 receive
optical signals whose phase difference corresponds to the phase
difference between two adjacent bits. That is to say, the first
photodiode 401 receives the first optical signal with the normal phase
from the first output terminal of the 1-bit delayed interferometer 200,
and outputs a positive signal. The second photodiode 402 receives a
second optical signal with the reversed phase from the second output
terminal of the 1-bit delayed interferometer 200, and outputs a
complementary signal.

[0051] The photoelectric current detection unit 430 detects photoelectric
currents which flow through the first photodiode 401 and the second
photodiode 402, respectively.

[0052] The differential transimpedance amplifier 410 is connected to each
output of the first photodiode 401 and the second photodiode 402, and the
positive signal is inputted from the first photodiode 401 and the
complementary signal is inputted from the second photodiode 402.
Moreover, the differential transimpedance amplifier 410 composes closed
feedback loops with feedback resisters 411 and 412. Here, as shown in
FIG. 2, an output amplifier 440 can be connected at the subsequent stage
of the differential transimpedance amplifier 410

[0053] In the present exemplary embodiment, the level adjustment unit 420
is configured as follows. That is to say, the level adjustment unit 420
adjusts the signal level in the closed feedback loop for the
complementary signal input of the differential transimpedance amplifier
410 (feedback resister 412) on the basis of the photoelectric current
generated by the normal phase optical signal in the first photodiode 401.
And, the level adjustment unit 420 adjusts the signal level in the closed
feedback loop for the positive signal input of the differential
transimpedance amplifier 410 (feedback resister 411) on the basis of the
photoelectric current generated by the reversed phase optical signal in
the second photodiode 402.

[0054] Next, the configuration of the optical receiver 400 according to
the present exemplary embodiment will be described more specifically. The
differential transimpedance amplifier 410 is provided with a differential
amplifier 413 which inputs the positive signal and the complementary
signal, and an emitter-follower circuit 414 (or source-follower circuit)
which is connected to the output of the differential amplifier 413. The
closed feedback loops are configured with the feedback resisters 411 and
412 which are connected between the output of the emitter-follower
circuit 414 and the input of the differential amplifier 413.

[0055] The level adjustment unit 420 is provided with a differential
circuit 415 which is connected between the output of the differential
amplifier 413 and the input of the emitter-follower circuit 414, and
adjusts the signal levels of positive and complementary signals in the
closed feedback loops respectively.

[0056] The photoelectric current detection unit 430 is provided with
current mirror circuits 431 and 432 outputting proportional currents
which are proportional to the photoelectric currents generated in the
first photodiode 401 and the second photodiode 402 respectively. And it
is configured so that the adjustment voltages can be inputted into the
input of the differential circuit 415 in reverse between positive and
complementary signals by resisters 433 and 434 as an adjustment voltage
generation unit which generates an adjustment voltage according to the
proportional current. That is to say, the adjustment voltage (generated
in the resister 433), which is based on the positive signal outputted by
the first photodiode 401, is inputted into the side which is connected to
a complementary signal output of the differential amplifier 413 in the
differential circuit 415 composing the level adjustment unit 420. On the
other, the adjustment voltage (generated in the resister 434), which is
based on the complementary signal outputted from the second photodiode
402, is inputted into the side which is connected to a positive signal
output of the differential amplifier 413 in the differential circuit 415
composing the level adjustment unit 420. Here, electrical resistance
values of the resisters 433 and 434 are set equal.

[0057] Next, the operation of the optical receiver 400 according to the
present exemplary embodiment will be described. FIG. 3 is a circuit
diagram to illustrate the operation of the optical receiver 400 according
to the present exemplary embodiment. The configuration of the optical
receiver 400 is the same as that shown in FIG. 2.

[0058] If there is no difference in the intensity between the first
optical signal received by the first photodiode 401 and the second
optical signal received by the second photodiode 402, the same electric
currents (α IPD1=α IPD2) flow through two current
mirror circuits 431 and 432 in the photoelectric current detection unit
430. Therefore, the same voltages (VPD1=VPD2) are generated by
the resisters 433 and 434, and the same voltages are inputted into the
level adjustment unit 420. Accordingly, in this case, the positive and
complementary signals are demodulated, amplified, and outputted without
any change (OUT and OUTB).

[0059] If there is the difference in the intensity between the first
optical signal and the second optical signal, the difference in the
current arises between the photoelectric current flowing through the
first photodiode 401 (IPD1) and the photoelectric current flowing
through the second photodiode 402 (IPD2), for example,
IPD1<IPD2. Therefore, electric currents (α
IPD1<α IPD2) and voltages (VPD1<VPD2)
corresponding to the difference in the current flowing through the
photodiodes are generated in the photoelectric current detection unit
430.

[0060] According to the configuration of the optical receiver 400 of the
present exemplary embodiment, the higher voltage (VPD2) is applied
to the side which is connected to the first photodiode 401 generating the
smaller photoelectric current (IPD1) in the differential circuit 415
composing the level adjustment unit 420. On the other hand, the lower
voltage (VPD1) is applied to the side which is connected to the
second photodiode 402 generating the larger photoelectric current
(IPD1). As a result, the difference in the intensity between the
first optical signal and the second optical signal is automatically
corrected, and the demodulated waveform is outputted. FIG. 5 shows signal
waveforms after RZ-DPSK demodulation. As mentioned above, even if the
difference in the intensity arises between the optical signals before
demodulation, according to the optical receiver 400 of the present
exemplary embodiment, it is possible to correct automatically the
difference in the received intensity and obtain the demodulated signal
waveform with good quality.

[0061] While the case using the bipolar transistors is shown in FIG. 2 and
FIG. 3, not limited to this, it is also possible to use a Field Effect
Transistor (FET) such as the MOS (Metal Oxide Semiconductor) type and the
like.

[0062] As described above, the optical receiver 400 of the present
exemplary embodiment has the configuration where the photoelectric
currents generated by the photodiodes are detected by the photoelectric
current detection unit 430 and are fed back to the level adjustment unit
420. As a result, the signal levels of positive and complementary signals
in two closed feedback loops of the differential transimpedance amplifier
410 are adjusted automatically. Therefore, it becomes possible to correct
automatically the difference in the intensity between two optical signals
before demodulation and to amplify the signals.

The Third Exemplary Embodiment

[0063] Next, the third exemplary embodiment of the present invention will
be described. FIG. 4 is a circuit diagram showing the configuration of an
optical receiver 500 according to the third exemplary embodiment of the
present invention. The optical receiver 500 includes a first photodiode
501, a second photodiode 502, a differential transimpedance amplifier
510, a level adjustment unit 520, and a photoelectric current detection
unit 530. Here, the optical receiver 500 composes an optical reception
device with the 1-bit delayed interferometer 200. In the optical receiver
500 according to the present exemplary embodiment, the configurations of
the level adjustment unit 520 and the photoelectric current detection
unit 530 are different from those of the optical receiver 400 according
to the second exemplary embodiment.

[0064] The first photodiode 501 and the second photodiode 502 receive
optical signals whose phase difference corresponds to the phase
difference between two adjacent bits. That is to say, the first
photodiode 501 receives a first optical signal with the normal phase from
the first output terminal of the 1-bit delayed interferometer 200, and
outputs a positive signal. The second photodiode 502 receives a second
optical signal with the reversed phase from the second output terminal of
the 1-bit delayed interferometer 200, and outputs a complementary signal.

[0065] The photoelectric current detection unit 530 detects photoelectric
currents which flow through the first photodiode 501 and the second
photodiode 502, respectively.

[0066] The differential transimpedance amplifier 510 is connected to each
output of the first photodiode 501 and the second photodiode 502, and the
positive signal is inputted from the first photodiode 501 and the
complementary signal is inputted from the second photodiode 502.
Moreover, the differential transimpedance amplifier 510 composes closed
feedback loops with feedback resisters 511 and 512. Here, as shown in
FIG. 4, an output amplifier 540 can be connected at the subsequent stage
of the differential transimpedance amplifier 510.

[0067] In the present exemplary embodiment, the level adjustment unit 520
is configured as follows. That is to say, the level adjustment unit 520
adjusts the signal level in the feedback loop for the input of the
positive signal of the differential transimpedance amplifier 510
(feedback register 511) on the basis of the reversed value of the voltage
generated according to the photoelectric current which is generated in
the first photodiode 501 by the normal phase optical signal. And, the
level adjustment unit 520 adjusts the signal level in the feedback loop
for the input of the complementary signal of the differential
transimpedance amplifier 510 (feedback register 512) on the basis of the
reversed value of the voltage generated according to the photoelectric
current which is generated in the second photodiode 502 by the reversed
phase optical signal.

[0068] Next, the configuration of the optical receiver 500 according to
the present exemplary embodiment will be described more specifically. The
differential transimpedance amplifier 510 is provided with a differential
amplifier 513 which inputs the positive signal and the complementary
signal, and an emitter-follower circuit 514 (or, source-follower circuit)
which is connected to the output of the differential amplifier 413. The
closed feedback loops are configured with the feedback resisters 511 and
512 which are connected between the output of the emitter-follower
circuit 514 and the input of the differential amplifier 513.

[0069] The level adjustment unit 520 is provided with a differential
circuit 515 which is connected between the output of the differential
amplifier 513 and the input of the emitter-follower circuit 514, and
adjusts the signal levels of positive and complementary signals in the
closed feedback loops respectively.

[0070] The photoelectric current detection unit 530 is provided with
current mirror circuits 531 and 532 outputting proportional currents
which are proportional to the photoelectric currents generated in the
first photodiode 501 and the second photodiode 502 respectively.
Furthermore, the photoelectric current detection unit 530 includes
resisters 533 and 534 as an adjustment voltage generation unit which
generates an adjustment voltage according to the proportional current,
and an inverter circuit 535 outputting a level adjustment voltage which
is obtained by inverting the adjustment voltage and amplifying it. And
the level adjustment voltage is inputted into the differential circuit
515 which composes the level adjustment unit 520. That is to say, the
adjustment voltage (generated in the resister 533), which is based on the
positive signal outputted by the first photodiode 501, is inverted by the
inverter circuit 535, and is inputted into the side which is connected to
a positive signal output of the differential amplifier 513 in the
differential circuit 515 composing the level adjustment unit 520. On the
other, the adjustment voltage (generated in the resister 534), which is
based on the complementary signal outputted from the second photodiode
502, is inverted by the inverter circuit 535, and is inputted into the
side which is connected to a complementary signal output of the
differential amplifier 513 in the differential circuit 515 composing the
level adjustment unit 520. Here, electrical resistance values of the
resisters 533 and 534 are set equal.

[0071] Next, the operation of the optical receiver 500 according to the
present exemplary embodiment will be described. If there is no difference
in the intensity between the first optical signal received by the first
photodiode 501 and the second optical signal received by the second
photodiode 502, the same electric currents flow through two current
mirror circuits 531 and 532 in the photoelectric current detection unit
530. Therefore, the level adjustment voltages of the outputs of the
inverter circuit 535 become equal, and the equal voltage is inputted into
the differential circuit 515 composing the level adjustment unit 520.
Accordingly, in this case, the positive and the complementary signals are
demodulated, amplified, and outputted without any change (OUT and OUTB).

[0072] If there is the difference in the intensity between the first
optical signal and the second optical signal, the difference in the
current arises between the photoelectric current flowing through the
first photodiode 501 and the photoelectric current flowing through the
second photodiode 502. At this time, the inverter circuit 535 outputs the
level adjustment voltages which are obtained by inverting the adjustment
voltages corresponding to the currents which flow through the photodiodes
and amplifying them. And then, the level adjustment voltages are inputted
into the differential circuit 515 composing the level adjustment unit
520.

[0073] As mentioned above, according to the configuration of the optical
receiver 500 of the present exemplary embodiment, the inverter circuit
535 outputs the level adjustment voltages depending on an amount to
correct the difference in the intensity between the first optical signal
and the second optical signal. Therefore, the difference in the intensity
between the first optical signal and the second optical signal is
automatically corrected. FIG. 5 shows signal waveforms after RZ-DPSK
demodulation. As described clearly in FIG. 5, even if the difference in
the intensity arises between the optical signals before demodulation,
according to the optical receiver 500 of the present exemplary
embodiment, it is possible to correct automatically the difference in the
received intensity and obtain the demodulated signal waveform with good
quality.

[0074] While the case using the bipolar transistors is shown in FIG. 4,
not limited to this, it is also possible to use field effect transistors
such as the MOS type and the like.

[0075] As described above, the optical receiver 500 of the present
exemplary embodiment has the configuration where the photoelectric
currents generated by the photodiodes are detected by the photoelectric
current detection unit 530 and are fed back to the level adjustment unit
520. As a result, the signal levels of positive and complementary signals
in two closed feedback loops of the differential transimpedance amplifier
510 are adjusted automatically. Therefore, it becomes possible to correct
automatically the difference in the intensity between two optical signals
before demodulation and to amplify the signals.

The Fourth Exemplary Embodiment

[0076] Next, the fourth exemplary embodiment of the present invention will
be described. In the present exemplary embodiment, a case using the
coherent detection system will be described. FIG. 6 is a block diagram
showing the configuration of a coherent optical reception device
according to the fourth exemplary embodiment of the present invention.
FIG. 6 shows a part of the coherent optical reception device which is
related to one polarization (polarization X). A coherent optical
reception device 10000 includes a coherent optical receiver 1000 and an
optical 90 degrees hybrid circuit 1100.

[0077] The optical 90 degrees hybrid circuit 1100 is provided with an
optical phase shifter 1101 and an optical mixer 1102. The optical 90
degrees hybrid circuit 1100 inputs an optical reception signal 1001 and a
first local oscillation light 1002, whose wave length is almost the same
as that of the optical reception signal 1001, from a local oscillation
light source. Here, the optical reception signal 1001 is the
demultiplexed signal into X polarization or Y polarization by a
polarization beam splitter (PBS).

[0078] The optical 90 degrees hybrid circuit 1100 outputs a first
interference optical signal by making the optical reception signal 1001
interfere with the first local oscillation light 1002, and outputs a
second interference optical signal by making the optical reception signal
1001 interfere with a second local oscillation light whose phase is
reverse to that of the first local oscillation light 1002. Specifically,
in the case of carrying out the coherent reception of the optical signal
modulated by a quadrature phase shift keying (QPSK), as shown in FIG. 6,
the optical reception signal 1001 is split into four branches by an
optical coupler in the optical 90 degrees hybrid circuit 1100. Similarly,
the first local oscillation light 1002 is split into four branches by an
optical coupler, each phase is shifted by 0, π/2, π, or 3π/2,
and then each branch light is made interfere with the optical reception
signal 1001, respectively.

[0079] The coherent optical receiver 1000 includes a first photodiode
1210, a second photodiode 1220, a differential transimpedance amplifier
1300, a level adjustment unit 1400, and a photoelectric current detection
unit 1500. The output of the differential transimpedance amplifier 1300
is connected to an analog-to-digital conversion unit (ADC) 1600 and a
digital signal processing unit (DSP) 1700 via an amplifier circuit.

[0080] The first photodiode 1210 receives a first interference optical
signal 1110 from the optical 90 degrees hybrid circuit 1100, and outputs
a positive signal. The second photodiode 1220 receives a second
interference optical signal 1120, and outputs a complementary signal.

[0081] Into the differential transimpedance amplifier 1300, the positive
signal is inputted from the first photodiode 1210, and the complementary
signal is inputted from the second photodiode 1220. The differential
transimpedance amplifier 1300 is provided with a feedback resister 1310
composing a closed feedback loop for the positive signal input and a
feedback resister 1320 composing a closed feedback loop for the
complementary signal input, respectively.

[0082] The photoelectric current detection unit 1500 detects photoelectric
currents generated in the first photodiode 1210 and the second photodiode
1220 respectively. And then, the level adjustment unit 1400 adjusts a
signal level in the closed feedback loop on the basis of the output of
the photoelectric current detection unit 1500.

[0083] Next, the operation of the coherent optical reception device 10000
will be described. The optical reception signal 1001 and the first local
oscillation light 1002 are inputted into the optical 90 degrees hybrid
circuit 1100. The optical 90 degrees hybrid circuit 1100 makes the
optical reception signal 1001 interfere with the first local oscillation
light 1002 and outputs a first interference optical signal 1110. And, the
optical 90 degrees hybrid circuit 1100 makes the optical reception signal
1001 interfere with the second local oscillation light whose phase is
reverse to that of the first local oscillation light and outputs a second
interference optical signal 1120. The first interference optical signal
1110 and the second interference optical signal 1120 are inputted into
the first photodiode 1210 and the second photodiode 1220 respectively and
are converted into current intensity modulated signals by carrying out
the photoelectric conversion. The converted current signals are inputted
into the differential transimpedance amplifier 1300 which has negative
feedback loops, and are changed from current signals to voltage signals.
The differential transimpedance amplifier 1300 demodulates the input
signal by deriving the difference between two input signals, and outputs
two demodulation signals which have a positive and complementary
relationship (OUTP and OUTN).

[0084] At this time, if there is a difference in the intensity between the
first interference optical signal 1110 and the second interference
optical signal 1120, the difference appears as a difference between the
photoelectric current in the first photodiode 1210 and that in the second
photodiode 1220.

[0085] Here, the optical reception device 10000 of the present exemplary
embodiment has the configuration of detecting the photoelectric currents
generated by two photodiodes 1210 and 1220 by using the photoelectric
current detection unit 1500, and feeding them back to the level
adjustment unit 1400 connected to the differential transimpedance
amplifier 1300. By using the configuration, the signal levels of positive
and complementary signals in two closed feedback loops are adjusted and
the difference in the intensity between the first interference optical
signal 1110 and the second interference optical signal 1120 is corrected
automatically. As a result, two demodulated signals are obtained where
the difference in the intensity between two signals before the
demodulation is corrected. The demodulated signal is amplified by an
amplifier circuit, analog-to-digital converted by the analog-to-digital
conversion unit (ADC) 1600 connected to the subsequent stage, on which
the digital signal processing is performed such as the polarization
demultiplexing, the offset compensation for light source frequency, the
phase compensation, and the like in the digital signal processing unit
(DSP) 1700.

[0086]FIG. 9 shows signal waveforms after QPSK demodulation carried out
by the coherent optical reception device 10000 according to the present
exemplary embodiment. In this case, a bit rate is equal to 31.78911 Gb/s.
As shown clearly in the figure, even if the difference in the intensity
arises between the optical signals before the demodulation, it is
possible to correct automatically the difference in the intensity between
the optical signals, and to obtain the demodulated signal waveform with
good quality. FIG. 15 shows signal waveforms after the demodulation
carried out by a related coherent optical reception device, for
comparison. In this case, it is found from the figure that another
correction process is necessary for those demodulated signals.

[0087] As described above, according to the present exemplary embodiment,
in the coherent optical reception device, it becomes possible to correct
automatically the difference in the received intensity which arises from
the difference in the intensity and the optical path of the optical
signal. That is to say, it is possible to correct automatically the
deterioration of CMRR which arises in the photodiode, and to obtain the
QPSK demodulation signal with good quality.

The Fifth Exemplary Embodiment

[0088] Next, the fifth exemplary embodiment of the present invention will
be described. In the present exemplary embodiment, a case using the
coherent detection system will be described. FIG. 7 is a circuit diagram
showing the configuration of a coherent optical receiver 2000 according
to the fifth exemplary embodiment of the present invention. FIG. 7 shows
only a part which is related to an I channel (Ix) of one polarization
(polarization X) in the coherent optical receiver. The coherent optical
receiver 2000 includes a first photodiode 2210, a second photodiode 2220,
a differential transimpedance amplifier 2300, a level adjustment unit
2400, and a photoelectric current detection unit 2500. Here, the coherent
optical receiver 2000 composes a coherent optical reception device with
the optical 90 degrees hybrid circuit 1100.

[0089] Each of the first photodiode 2210 and the second photodiode 2220
receives an interference optical signal which is obtained by making an
optical reception signal interfere with a local oscillation light. That
is to say, the first photodiode 2210 receives a first interference
optical signal from the optical 90 degrees hybrid circuit 1100, and
outputs a positive signal. The second photodiode 2220 receives a second
interference optical signal from the optical 90 degrees hybrid circuit
1100, and outputs a complementary signal.

[0090] The photoelectric current detection unit 2500 detects photoelectric
currents which flow through the first photodiode 2210 and the second
photodiode 2220, respectively.

[0091] The differential transimpedance amplifier 2300 is connected to each
output of the first photodiode 2210 and the second photodiode 2220, and
the positive signal is inputted from the first photodiode 2210 and the
complementary signal is inputted from the second photodiode 2220.
Moreover, the differential transimpedance amplifier 2300 composes closed
feedback loops with feedback resisters 2310 and 2320.

[0092] In the present exemplary embodiment, the level adjustment unit 2400
is configured as follows. That is to say, the level adjustment unit 2400
adjusts the signal level in the closed feedback loop for the
complementary signal input of the differential transimpedance amplifier
2300 (feedback resister 2320) on the basis of the photoelectric current
generated by the first interference optical signal in the first
photodiode 2210. And, the level adjustment unit 2400 adjusts the signal
level in the closed feedback loop for the positive signal input of the
differential transimpedance amplifier 2300 (feedback resister 2310) on
the basis of the photoelectric current generated by the second
interference optical signal in the second photodiode 2220.

[0093] Next, the configuration of the coherent optical receiver 2000
according to the present exemplary embodiment will be described more
specifically. The differential transimpedance amplifier 2300 is provided
with a differential amplifier 2330 which inputs the positive signal and
the complementary signal, and an emitter-follower circuit 2340 (or
source-follower circuit) which is connected to the output of the
differential amplifier 2330. The closed feedback loops are configured
with the feedback resisters 2310 and 2320 which are connected between the
output of the emitter-follower circuit 2340 and the input of the
differential amplifier 2330.

[0094] The level adjustment unit 2400 is provided with a differential
circuit 2410 which is connected between the output of the differential
amplifier 2330 and the input of the emitter-follower circuit 2340, and
adjusts the signal levels of positive and complementary signals in the
closed feedback loops respectively.

[0095] The photoelectric current detection unit 2500 is provided with
current mirror circuits 2511 and 2512 outputting proportional currents
which are proportional to the photoelectric currents generated in the
first photodiode 2210 and the second photodiode 2220 respectively.
Although the case using current mirror circuits is described as an
example in the present exemplary embodiment, not limited to the case, it
is also possible to use a circuit other than the current mirror circuit
as long as the circuit outputs the proportional current which is
proportional to the inputted photoelectric current.

[0096] The coherent optical receiver 2000 of the present exemplary
embodiment is configured so that the adjustment voltages can be inputted
into the input of the differential circuit 2410 in reverse between
positive and complementary signals by resisters 2521 and 2522 as an
adjustment voltage generation unit which generates an adjustment voltage
according to the proportional current. That is to say, the adjustment
voltage (generated in the resister 2521), which is based on the positive
signal outputted by the first photodiode 2210, is inputted into the side
which is connected to a complementary signal output of the differential
amplifier 2330 in the differential circuit 2410 composing the level
adjustment unit 2400. On the other, the adjustment voltage (generated in
the resister 2522), which is based on the complementary signal outputted
from the second photodiode 2220, is inputted into the side which is
connected to a positive signal output of the differential amplifier 2330
in the differential circuit 2410 composing the level adjustment unit
2400.

[0097] Next, the operation of the coherent optical receiver 2000 according
to the present exemplary embodiment will be described. FIG. 8 is a
circuit diagram to illustrate the operation of the coherent optical
receiver 2000 according to the present exemplary embodiment. The
configuration of the coherent optical receiver 2000 is the same as that
shown in FIG. 7.

[0098] If there is no difference in the intensity between the first
interference optical signal received by the first photodiode 2210 and the
second interference optical signal received by the second photodiode
2220, the same electric currents (α IPD1=αIPD2)
flow through two current mirror circuits 2511 and 2512 in the
photoelectric current detection unit 2500. Therefore, the same voltages
(VPD1=VPD2) are generated by the resisters 2521 and 2522, and
the same voltages are applied to the level adjustment unit 2400.
Accordingly, in this case, the positive and complementary signals are
demodulated, amplified, and outputted without any change (OUTP and OUTN).

[0099] If there is the difference in the intensity between the first
interference optical signal and the second interference optical signal,
the difference in the current arises between the photoelectric current
flowing through the first photodiode 2210 (IPD1) and the
photoelectric current flowing through the second photodiode 2220
(IPD2), for example, IPD1<IPD2. Therefore, electric
currents (α IPD1<αIPD2) and voltages
(VPD1<VPD2) corresponding to the difference in the current
flowing through the photodiodes are generated in the photoelectric
current detection unit 2500.

[0100] According to the configuration of the coherent optical receiver
2000 of the present exemplary embodiment, the higher voltage (VPD2)
is applied to the side which is connected to the first photodiode 2210
generating the smaller photoelectric current (IPD1) in the
differential circuit 2410 composing the level adjustment unit 2400. On
the other hand, the lower voltage (VPD1) is applied to the side
which is connected to the second photodiode 2220 generating the larger
photoelectric current (IPD1). As a result, the difference in the
intensity between the first interference optical signal and the second
interference optical signal is automatically corrected, and the
demodulation waveform is outputted. FIG. 9 shows signal waveforms after
QPSK demodulation in the case that the difference in the intensity arises
between the optical signals before demodulation. In this case, a bit rate
is equal to 31.78911 Gb/s. As mentioned above, even if the difference in
the intensity arises between the optical signals before demodulation,
according to the optical receiver 2000 of the present exemplary
embodiment, it is possible to correct automatically the difference in the
received intensity and obtain the demodulated signal waveform with good
quality.

[0101] While the case using the bipolar transistors is shown in FIG. 7 and
FIG. 8, not limited to this, it is also possible to use a Field Effect
Transistor (FET) such as the MOS (Metal Oxide Semiconductor) type and the
like.

[0102] As described above, the coherent optical receiver 2000 of the
present exemplary embodiment has the configuration where the
photoelectric currents generated by the photodiodes are detected by the
photoelectric current detection unit 2500 and are fed back to the level
adjustment unit 2400. As a result, the signal levels of positive and
complementary signals in two closed feedback loops of the differential
transimpedance amplifier 2300 are adjusted automatically. Therefore, it
becomes possible to correct automatically the difference in the intensity
between two optical signals before demodulation and to amplify the
signals.

The Sixth Exemplary Embodiment

[0103] Next, the sixth exemplary embodiment of the present invention will
be described. In the present exemplary embodiment, a case using the
coherent detection system will be described. FIG. 10 is a circuit diagram
showing the configuration of a coherent optical receiver 3000 according
to the sixth exemplary embodiment of the present invention. FIG. 10 shows
only a part which is related to an I channel (Ix) of one polarization
(polarization X) in the coherent optical receiver. The coherent optical
receiver 3000 includes a first photodiode 3210, a second photodiode 3220,
a differential transimpedance amplifier 3300, a level adjustment unit
3400, and a photoelectric current detection unit 3500. Here, the coherent
optical receiver 3000 composes a coherent optical reception device with
the optical 90 degrees hybrid circuit 1100. In the coherent optical
receiver 3000 according to the present exemplary embodiment, the
configurations of the level adjustment unit 3400 and the optical current
detection unit 3500 are different from those of the coherent optical
receiver 2000 according to the fifth exemplary embodiment.

[0104] Each of the first photodiode 3210 and the second photodiode 3220
receives an interference optical signal which is obtained by making an
optical reception signal interfere with a local oscillation light. That
is to say, the first photodiode 3210 receives a first interference
optical signal from the optical 90 degrees hybrid circuit 1100, and
outputs a positive signal. The second photodiode 2220 receives a second
interference optical signal from the optical 90 degrees hybrid circuit
1100, and outputs a complementary signal.

[0105] The photoelectric current detection unit 3500 detects photoelectric
currents which flow through the first photodiode 3210 and the second
photodiode 3220, respectively.

[0106] The differential transimpedance amplifier 3300 is connected to each
output of the first photodiode 3210 and the second photodiode 3220, and
the positive signal is inputted from the first photodiode 3210 and the
complementary signal is inputted from the second photodiode 3220. The
differential transimpedance amplifier 3300 composes closed feedback loops
with feedback resisters 3310 and 3320.

[0107] In the present exemplary embodiment, the level adjustment unit 3400
is configured as follows. That is to say, the level adjustment unit 3400
adjusts the signal level in the closed feedback loop for the positive
signal input of the differential transimpedance amplifier 3300 (feedback
resister 3310) on the basis of the reversed value of the voltage
generated corresponding to the photoelectric current which is generated
in the first photodiode 3210 by the first interference optical signal.
And, the level adjustment unit 3400 adjusts the signal level in the
closed feedback loop for the complementary signal input of the
differential transimpedance amplifier 3300 (feedback resister 3320) on
the basis of the reversed value of the voltage generated corresponding to
the photoelectric current which is generated in the second photodiode
3220 by the second interference optical signal.

[0108] Next, the configuration of the coherent optical receiver 3000
according to the present exemplary embodiment will be described more
specifically. The differential transimpedance amplifier 3300 is provided
with a differential amplifier 3330 which inputs the positive signal and
the complementary signal, and an emitter-follower circuit 3340 (or,
source-follower circuit) which is connected to the output of the
differential amplifier 3330. The closed feedback loops are configured
with the feedback resisters 3310 and 3320 which are connected between the
output of the emitter-follower circuit 3340 and the input of the
differential amplifier 3330.

[0109] The level adjustment unit 3400 is provided with a differential
circuit 3410 which is connected between the output of the differential
amplifier 3330 and the input of the emitter-follower circuit 3340, and
adjusts the signal levels of positive and complementary signals in the
closed feedback loops respectively.

[0110] The photoelectric current detection unit 3500 is provided with
current mirror circuits 3511 and 3512 outputting proportional currents
which are proportional to the photoelectric currents generated in the
first photodiode 3210 and the second photodiode 3220 respectively.
Furthermore, the photoelectric current detection unit 3500 includes
resisters 3521 and 3522 as an adjustment voltage generation unit which
generates an adjustment voltage according to the proportional current,
and an inverter circuit 3530 outputting a level adjustment voltage which
is obtained by inverting the adjustment voltage and amplifying it. And
the level adjustment voltage is inputted into the differential circuit
3410 which composes the level adjustment unit 3400. That is to say, the
adjustment voltage (generated in the resister 3521), which is based on
the positive signal outputted by the first photodiode 3210, is inverted
by the inverter circuit 3530, and is inputted into the side which is
connected to a positive signal output of the differential amplifier 3330
in the differential circuit 3410 composing the level adjustment unit
3400. On the other, the adjustment voltage (generated in the resister
3522), which is based on the complementary signal outputted from the
second photodiode 3220, is inverted by the inverter circuit 3530, and is
inputted into the side which is connected to a complementary signal
output of the differential amplifier 3330 in the differential circuit
3410 composing the level adjustment unit 3400.

[0111] Next, the operation of the coherent optical receiver 3000 according
to the present exemplary embodiment will be described. If there is no
difference in the intensity between the first interference optical signal
received by the first photodiode 3210 and the second interference optical
signal received by the second photodiode 3220, the same electric currents
flow through two current mirror circuits 3511 and 3512 in the
photoelectric current detection unit 3500. Therefore, the level
adjustment voltages of the outputs of the inverter circuit 535 become
equal, and the equal voltage is inputted into the differential circuit
3410 composing the level adjustment unit 3400. Accordingly, in this case,
the positive and the complementary signals are demodulated, amplified,
and outputted without any change (OUTP and OUTN).

[0112] If there is the difference in the intensity between the first
optical signal and the second optical signal, the difference in the
current arises between the photoelectric current flowing through the
first photodiode 3210 and the photoelectric current flowing through the
second photodiode 3220. At this time, the inverter circuit 3530 outputs
the level adjustment voltages which are obtained by inverting the
adjustment voltages corresponding to the currents which flow through the
photodiodes and amplifying them. And then, the level adjustment voltages
are inputted into the differential circuit 3410 composing the level
adjustment unit 3400.

[0113] As mentioned above, according to the configuration of the optical
receiver 3000 of the present exemplary embodiment, the inverter circuit
3530 outputs the level adjustment voltages depending on an amount to
correct the difference in the intensity between the first interference
optical signal and the second interference optical signal. Therefore, the
difference in the intensity between the first interference optical signal
and the second interference optical signal is automatically corrected.
FIG. 9 shows signal waveforms after the QPSK demodulation in the case
that the difference in the intensity arises between the optical signals
before demodulation. In this case, a bit rate is equal to 31.78911 Gb/s.
As described clearly in FIG. 9, even if the difference in the intensity
arises between the optical signals before demodulation, according to the
coherent optical receiver 3000 of the present exemplary embodiment, it is
possible to correct automatically the difference in the received
intensity and obtain the demodulated signal waveform with good quality.

[0114] While the case using the bipolar transistors is shown in FIG. 10,
not limited to this, it is also possible to use field effect transistors
such as the MOS type and the like.

[0115] As described above, the level adjustment unit 3400 is provided with
the differential circuit 3410 which is connected between the output of
the differential amplifier 3330 and the input of the emitter-follower
circuit 3340. However, not limited to this, it is also possible to use
the level adjustment unit 3400 which is provided with a differential
circuit 3420 connected to the output part of the emitter-follower circuit
3340, as shown in FIG. 11. Even in this case, the level adjustment unit
3400 can adjust the signal levels of positive and complementary signals
in the closed feedback loops of the differential transimpedance amplifier
3300.

[0116] As described above, the coherent optical receiver 3000 of the
present exemplary embodiment has the configuration where the
photoelectric currents generated by the photodiodes are detected by the
photoelectric current detection unit 3500 and are fed back to the level
adjustment unit 3400. As a result, the signal levels of positive and
complementary signals in two closed feedback loops of the differential
transimpedance amplifier 3300 are adjusted automatically. Therefore, it
becomes possible to correct automatically the difference in the intensity
between two optical signals before demodulation and to amplify the
signals.

[0117] The present invention is not limited to the above-mentioned
exemplary embodiments and can be variously modified within the scope of
the invention described in the claims. It goes without saying that these
modifications are also included in the scope of the invention.

[0118] This application is based upon and claims the benefit of priority
from Japanese patent application No. 2010-097624, filed on Apr. 21, 2010,
the disclosure of which is incorporated herein in its entirety by
reference.

DESCRIPTION OF THE CODES

[0119] 100 optical reception device

[0120] 200 1-bit delayed interferometer

[0121] 210 optical input signal

[0122] 221 first optical signal

[0123] 222 second optical signal

[0124] 300, 400, 500 optical receiver

[0125] 301, 401, 501 first photodiode

[0126] 302, 402, 502 second photodiode

[0127] 310, 410, 510 differential transimpedance amplifier

[0128] 311, 312, 411, 412, 511, 512 feedback resister

[0129] 320, 420, 520 level adjustment unit

[0130] 330, 430, 530 photoelectric current detection unit

[0131] 440, 540 output amplifier

[0132] 413, 513 differential amplifier

[0133] 414, 514 emitter-follower circuit

[0134] 415, 515 differential circuit

[0135] 431, 432, 531, 532 current mirror circuit

[0136] 433, 434, 533, 534 resister

[0137] 535 inverter circuit

[0138] 600, 700 related optical reception device

[0139] 610, 710 related optical receiver

[0140] 611, 612, 711, 712 photodiode (PD)

[0141] 620, 720 transimpedance amplifier

[0142] 622, 722 negative feedback

[0143] 650 1-bit delayed interferometer

[0144] 651 optical input signal

[0145] 652, 653 optical signal

[0146] 721 differential amplifier

[0147] 730 level adjustment unit

[0148] 10000 coherent optical reception device

[0149] 1000, 2000 coherent optical receiver

[0150] 1001 optical reception signal

[0151] 1002 first local oscillation light

[0152] 1100 optical 90 degrees hybrid circuit

[0153] 1101 optical phase shifter

[0154] 1102 optical mixer

[0155] 1110 first interference optical signal

[0156] 1120 second interference optical signal

[0157] 1210, 2210, 3210 first photodiode

[0158] 1220, 2220, 3220 second photodiode

[0159] 1300, 2300, 3300 differential transimpedance amplifier

[0160] 1310, 1320, 2310, 2320, 3310, 3320 feedback resister

[0161] 1400, 2400, 3400 level adjustment unit

[0162] 1500, 2500, 3500 photoelectric current detection unit

[0163] 1600 analog-to-digital conversion unit (ADC)

[0164] 1700 digital signal processing unit (DSP)

[0165] 2330, 3330 differential amplifier

[0166] 2340, 3340 emitter-follower circuit

[0167] 2410, 3410 differential circuit

[0168] 2511, 2512, 3511, 3512 current mirror circuit

[0169] 2521, 2522, 3521, 3522 resister

[0170] 3530 inverter circuit

[0171] 5000 related coherent optical reception device

[0172] 5001 optical reception signal

[0173] 5002 local oscillation light

[0174] 5010 polarization separation unit

[0175] 5100 optical 90 degrees hybrid circuit

[0176] 5200 photodiode

[0177] 5300 differential transimpedance amplifier

[0178] 5400 analog-to-digital conversion unit (ADC)

[0179] 5500 digital signal processing unit (DSP)

Patent applications by Yasuyuki Suzuki, Tokyo JP

Patent applications by NEC Corporation

Patent applications in class Determination of communication parameter

Patent applications in all subclasses Determination of communication parameter